This invention relates to a miniature synchronous motor, more particularly to a self-starting miniature synchronous motor.
Although various types of motors of the kind described above have been, a satisfactory one is not yet available which can self-start always in a definite direction and can produce relatively large torque. For example, according to a synchronous motor disclosed in U.S. Pat. No. 3,256,453 N and S magnetic poles are alternately arranged about the periphery of the rotor and a plurality of stator arms each having a pole face are disposed to surround the rotor. With this construction, however, since the number of pairs of poles is an even number, for example 8, when one of the stator main poles is magnetized to produce a repulsive force between it and an opposed rotor pole (for example N pole), the other stator main pole which is diametrically opposite to said one stator main pole does not produce any repulsive force between it and the other rotor pole (N pole) which is diametrically opposite to said rotor pole but creates a attractive force between it and a rotor pole (S pole) adjacent said other rotor pole. In other words, the rotor torque is not produced by diametrically opposite rotor poles so that the rotor shaft will be subjected to side pressure which causes one sided wear of the shaft as well as shortening of the motor life. Furthermore, with this construction, one pair of rotor poles is always idle thus decreasing the efficiency of utilization of the poles.
According to another example disclosed in U.S. Pat. No. 3,984,972, the motor comprises a rotor including a diametrically opposite poles and a stator having two semicircular opposing portions encircling the rotor, the semicircular opposing portions being disposed in a point symmetrical relationship with respect to the rotor shaft. With this construction, however, if the frequency of the stepping pulse supplied to a coil common to stator portions were too high, when the rotor rotates by being supplied with a driving pulse, before the rotor stops at a stable position due to rotor inertia the next driving pulse would be applied thereby resulting in a hunting. Furthermore, as this motor is of the stepping pulse drive type if it is energized by an alternating current pulse having no zero amplitude interval between adjacent driving pulses the rotor will often assume stable stopping state in which the rotor does not rotate. For this reason, the motor of this construction could be operated only with a stepping pulse having a relatively low frequency.